The present invention relates in general to amplifier circuits and, more particularly, to an output stage of an amplifier.
Differential amplifier circuits are used in a myriad of applications to amplify differential signals and provide a single-ended output signal. For example, in U.S. Pat. No. 5,153,529, the differential input signal is applied to a conventional differential transistor pair. A differential to single-ended converter converts the differential signal to single-ended operation. The single-ended signal is processed through an output stage that provides additional gain and output drive.
The amplifier circuit requires one or more power supply potentials for operation. The modern trend is to lower the operating potential to the amplifier, especially for battery-supplied applications such as cellular telephones, digital pagers and portable computers. The lower operating potential is desirable as it requires fewer serially coupled battery cells to supply the operating potential to the end use circuit.
The input impedance looking into the output stage should be kept as high as possible to provide maximum gain and drive capacity for the amplifier. In the prior art, transistors are often cascoded between the input of the output stage and the power supply conductor to provide the high impedance at that node. Unfortunately, a low power supply operating potential of say 1.0 volts does not allow sufficient headroom for cascoded transistors to provide the high input impedance.
Hence, a need exists to maintain high impedance into the output stage with a low power supply potential to the amplifier.